Throughout the world, telephone systems and communication systems rely on a battery feed circuit or power supply to provide electrical power to a subscriber line or load. The level of power supplied to the subscriber line must be controlled according to regional specifications such as Bellcore, CCITT, and EIA to ensure proper operation of the telecommunication equipment attached to the subscriber line. The power supply or battery feed circuit generally provides direct current (DC) power within an acceptable range of voltage and current levels to the subscriber line. The battery feed circuit should ideally provide an appropriate amount of power for both long subscriber lines or loops and short subscriber lines or loops. For example, more power is required for the higher impedance loops such as long loops than for the lower impedance loops such as short loops.
Most telephone or communication systems include a central office (office exchange) which generally includes a line card having a subscriber line interface circuit (SLIC device) and a subscriber line audio-processing circuit (such as a SLAC.TM. or DSLAC.TM. device manufactured by Advanced Micro Devices). The SLIC device is an analog interface coupled to the subscriber line; the SLIC device generally includes audio amplifiers and often supports line card "BORSHT" (battery feed, over-voltage protection, ringing signal, supervision of the line, hybrid two to four-wire conversion, and test) functions. The SLIC device often is coupled to SLAC device on the line card. The subscriber line is generally coupled to a subscriber telephone, termination equipment or other customer communications equipment.
The SLAC device which is generally coupled to the SLIC device transmits and receives analog signals from the SLIC device and provides digital signals to the central office. The SLAC device generally includes analog-to-digital (A/D) converters and digital-to-analog (D/A) converters. The SLAC device typically handles filtering and conversion tasks necessary to interface the analog signals from the SLIC device to the digital pulse code modulation (PCM) highway associated with the central office.
Generally, the SLIC device or other equipment in the central office includes a battery feed circuit or DC power supply which provides power to the subscriber line. Alternatively, the battery feed circuit can be an external component. The battery feed circuit often includes an analog regulator or other analog control circuit which provides an appropriate current level or voltage level to the subscriber line in accordance with the regional specifications. The battery feed circuit requires the analog regulator to provide the appropriate output current level or voltage level over the variable impedance of the subscriber liner. The impedance of the subscriber line can vary from less than 200 ohms to more than 3.1 k ohms. The variance in impedance is generally due to the variable subscriber line lengths from the central office to the customer telephone or other equipment.
Analog regulation circuits are disadvantageous because they consume a relatively large amount of power, generate relatively significant amounts heat, occupy space on the integrated circuit substrate and on the line card, and are subject to long-term instability and imprecision. Such disadvantages can be accentuated by the large impedance variation associated with the subscriber line in a telephone system. Thus, there is a need for a battery feed control circuit which is inexpensive, occupies a minimal amount of chip space, and is precise and stable.